Deflection coil system for colour television

ABSTRACT

A deflection coil system for colour television, comprising a ring core which is divided into four quarters by a horizontal and a vertical deflection plane. Each of the quarters is provided with a recess near the front of the ring core so as to reduce raster distortion.

United States Patent Talsma Oct. 14, 1975 DEFLECTION COIL SYSTEM FOR COLOUR [56] References Cited TELEVISION UNITED STATES PATENTS [75] Inventor: Tjitte Talsma, Eindhoven, 2,821,671 1/1958 Kratz et a1. 335/213 Netherlands 3,731,241 5 1973 Coupland 335 213 [73] Assignee: U.S. Philips Corporation, New FOREIGN PATENTS OR APPLICATIONS York, N.Y. 2,908 9/1964 Japan 335/210 [22] Filed: 1974 Primary ExaminerHa.rold Broome [21] App]. No.: 432,304 Attorney, Agent, or FirmFrank R. Trifari; Henry 1.

Steckler [30] Foreign Apphcatlon Priority Data [57] ABSTRACT Feb. 19, 1973 Netherlands 7302257 A deflection coil System for Colour television, com prising a ring core which is divided into four quarters (51.2 335/2l(l){,03lI; 5/72/(l)(3; y a horizontal and a vertical deflection plane. Each [58] Field 21 l 21} of the quarters is provided with a recess near the front of the ring core so asfto reduce raster distortion.

3 Claims, 5 Drawing Figures US. Patent Oct. 14, 1975 DEFLECTION COIL SYSTEM FOR COLOUR TELEVISION The invention relates to a deflection coil system for a colour television display tube having a partly flared outer surface, the said tube comprising means for generating three electron beams which are situated substantially in one plane, the said deflection coil system comprising a first pair of saddle-like deflection coils which are adapted to the flared shape and which are arranged on both sides of a first deflection plane which, after the mounting of the deflection coilsystem on the display tube, substantially coincides with the plane of the electron beams, a second pair of saddle-like deflection coils which are adapted to the flared shape" and which are arranged on both sides of a second-deflection plane which is perpendicular to the first deflection plane, and a flared ferromagnetic ring core which encloses both coil pairs and which is divided into four quarters by the two deflection planes.

Colour television display tubes which are adapted to generate three electron beams which are situated in one plane offer the advantage that the three beams can be converged using simpler means than in the case of display tubes in which the three beams are regularly distributed over the circumference of a circle. There fore, the deflection fields generated by the two deflection coil pairs must satisfy given requirements. This can be achieved by a suitable choice of the shape and the distribution of turns of these coils, but it was found in practice that these coils give rise to serious raster distortion: if a raster consisting of horizontal and vertical lines is'displayed on the display screen of a display tube equipped with such a deflection coil system, it appears that the ends of notably the upper and lower'h'orizontal lines of the raster are deflected towards the centre of the display screen. This defect occurs, at least'partly, because the deflection field generated by the image coils (deflection coils for deflection in the vertical direction) on the front of the deflection coil system, i.e., where the diameter of the ring core is largest, has a bar rel-like shape. This means that the magnetic field strength which would have to be only horizontally directed for the vertical deflection, also contains a vertical component which is largest at the areas which are furthest removed from the deflection planes.

The invention has for its object to provide a deflection coil system in which this defect is substantially reduced. To this end, the invention is characterized in that the ring core is provided with a recess in each of the four quarters at the area where its diameter is largest.

Consequently, the field of the vertical deflection coils is substantially attenuated at the area where the vertical component of the field strength is the largest, with the result that the raster distortion is substantially reduced.

The shape, the size and the location of the four recesses are preferably chosen such that the ring core is symmetrical with respect to the two deflection planes.

The invention will be described in detail hereinafter with reference to the drawing.

FIG. 1 is a diagrammatic longitudinal sectional view of a colour television display tube provided with a deflection coil system,

FIG. 2 is a front view of a display screen of the tube shown in FIG. 1 so as to illustrate the raster distortion,

FIG. 3 is a diagrammatic view of the vertical deflection field in the foremost part of the deflection coil system shown in FIG. 1,

FIG. 4 is a perspective view of a ring core of a deflection coil system according to the invention, and

FIG. 5 is a front view of the ring core shown in FIG. 4.

FIG. 1 is a diagrammatic longitudinal sectional view of a display tube 1 for colour television. This tube consists of a cylindrical neck portion 3 and an adjoining flared portion 5 which is closed on the front side (at the right in FIG. 1) by a display screen 7. Arranged in the neck portion 3 are three electron guns 9 which are situated in a plane perpendicular to the plane of the drawing and which are capable of generating electron beams which are emitted in this plane. Provided on the tube 1, at the area where the neck portion 3 changes over into the flared portion 5, is a deflection coil system 11 which coaxially encloses the tube and which consists of a first pair ofsaddle-like deflection coils 13 for deflection of the electron beamsin the horizontal direction, a second pair of saddle-like deflection coils 15 for deflection of the electron beams in. the vertical direction, and a ring core 17 which encloses both coil pairs. As is shown in FIG. 1, the shape of the deflection coils l3 and 15 is adapted to the flared shape of the display tube 1. The horizontal deflection coils 13 are situated on both sides of a horizontal deflection plane which coincides with the said planein which the three electron beams are emitted. The vertical deflection coils 15 are situated on both sides of a vertical deflection plane which is perpendicular to the horizontal deflection plane and which, consequently, coincides with the plane of the drawing.

The ring core 17 is made of a ferromagnetic material, preferably ferrite. It is flared .in the forward direction so that it fits aboutthe deflection coil pairs 13, 15 with little play. I

The requirements to be satisfied by the distribution of turns and the shape of the deflectioncoils 13, 15 so as to achieve proper convergence of the electron beams generated by theelectron guns.9, using simple means, also when the beams are deflected so that they are incident on the display screen 7 at an arbitrary location, are known and they will not elaborated herein. However, it was found that a raster of horizontal and vertical lines which is displayed on the display screen of a display tube equipped with such a deflection coil system is distorted. So as to illustrate this phenomenon, FIG. 2 shows a front view of a display screen 7 with a part of such a raster, i.e., the central horizontal line 19, a horizontal line 21 which is situated near the top of the display screen, and a horizontal line 23 which is situated near the bottom of the screen. The other lines of the raster have been omitted for the sake of clarity. As can be readily seen, the central line 19 extends substantially horizontally over its entire length, whilst the upper line 21 and the lower line 23 are bent towards the centre of the display screen 7 near their ends. The ends of these lines have shifted towards the centre over a distance d. Measurements performed on display tubes having a display screen dimensioned 52 X 39 cm revealed a value of 12 mm for d. This value is so high, that it is very difficult to correct the defect completely by adaptation of the deflection currents in the deflection coils 13, 15. Complex and expensive circuits would be required for this purpose.

An explanation of the described raster distortion can be found by considering the magnetic field of the vertical deflection coils at the area of the front of the ring core 17 (the side having the largest diameter). This field is as shown in FIG. 3. This figure shows a section of the ring core 17 which is taken along the line III-III in FIG. 1, a number of lines of force 25 of the magnetic 29, with the result that at this area a substantially purely vertical deflection of the electron beams takes place. The further removed from the lines 27 and 29, however,.the more curved the lines of force 25 are, which means that at these areas the magnetic field strength not only contains a horizontal but also a vertical component, so that the electron beams are also horizontally deflected to a given extent. This horizontal deflection gives rise to-the described raster distortion.

It was found that the raster distortion can be substantially reduced by attenuating the deflection field at the area where the vertical component is largest. To this end, according to the invention, recesses 31 are provided in the ring core 17 at these areas as shown in FIG. 4. Because a region with substantially'curved magnetic lines of force 25 occurs in each of the four quarters of the ring core 17, each quarter must be provided with a recess 31. By a suitable choice of the location and the dimensions of the recesses, it can be ensured that the residual raster distortion is minimized. FIG. 5 diagrammatically shows which parameters are of importance in this respect. FIG. 5 is a diagrammatic front view of a ring core 17 comprising four recesses, the largest width of which is b and the largest depth h. The connecting line between the axis of the ring core 17 and the centre of the recess encloses an angle a with the horizontal deflection plane. It was found that the best results are obtained if the values of h, b and a are the same for all four recesses 31, which means that the ring core 17 with the recesses 31 is symmetrical with respect to the two deflection planes. In the display tube and the deflection coil system which initially produced the raster distortion described with reference to FIG. 2, recesses were provided which were dimensioned as follows: a 35, b 50 mm and h 15 mm. The value of d, originally 12 mm, was thus reduced to 4 mm, which can be further corrected using known and comparatively simple means for influencing the deflection current.

In order to maintain. the geometrical shape of the original ring core 17, the recesses 31 can be filled with a suitable, non-ferromagnetic material, for example, epoxy resin.

What is claimed is:

1. A deflection coil system for a colour television display tube having a partly flared outer surface and means for generating three electron beams which are situated substantially in one plane, the said deflection coil system comprising a first pair of saddle-like deflection coils which are adapted to the flared shape and which are arranged on both sides of a first deflection plane which, after mounting of the deflection coil system on the display tube, substantially coincides with the plane of the electron beams, a second pair of saddle-like deflection coils which are adapted to the flared shape and which are arranged on both sides of a second deflection plane which is perpendicular to the first de-' flection plane, a flared ferromagnetic ring core which encloses both coil pairs and which is divided into four quarters by the two deflection planes, and means for correcting raster distortion comprising the ring core provided with a recess in each of the four quarters at the area where its diameter is largest.

2.,A deflection coil system as claimed in claim 1, characterized in that the shape, the dimensions and the location of the four recesses are chosen 'to be such that the ring core is symmetrical with respect to the two deflection planes.

3. A deflection coil system as claimed in claim 1, characterized in that the recesses are filled with a non-ferromagnetic solid matter. 

1. A deflection coil system for a colour television display tube having a partly flared outer surface and means for generating three electron beams which are situated substantially in one plane, the said deflection coil system comprising a first pair of saddle-like deflection coils which are adapted to the flared shape and which are arranged on both sides of a first deflection plane which, after mounting of the deflection coil system on the display tube, substantially coincides with the plane of the electron beams, a second pair of saddle-like deflection coils which are adapted to the flared shape and which are arranged on both sides of a second deflection plane which is perpendicular to the first deflection plane, a flared ferromagnetic ring core which encloses both coil pairs and which is divided into four quarters by the two deflection planes, and means for correcting raster distortion comprising the ring core provided with a recess in each of the four quarters at the area where its diameter is largest.
 2. A deflection coil system as claimed in claim 1, characterized in that the shape, the dimensions and the locAtion of the four recesses are chosen to be such that the ring core is symmetrical with respect to the two deflection planes.
 3. A deflection coil system as claimed in claim 1, characterized in that the recesses are filled with a non-ferromagnetic solid matter. 